U.S. patent application number 16/516778 was filed with the patent office on 2020-02-27 for exhaust gas control apparatus for internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Norihisa NAKAGAWA, Shogo TANAKA.
Application Number | 20200063630 16/516778 |
Document ID | / |
Family ID | 67539280 |
Filed Date | 2020-02-27 |
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United States Patent
Application |
20200063630 |
Kind Code |
A1 |
TANAKA; Shogo ; et
al. |
February 27, 2020 |
EXHAUST GAS CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE
Abstract
An exhaust gas control apparatus includes an exhaust gas control
catalyst disposed in an exhaust passage, a filter disposed
downstream of the catalyst, a secondary air supply device
configured to supply secondary air into exhaust gas flowing into
the filter at a location downstream of the catalyst in an exhaust
gas flow direction, and an electronic control unit. The electronic
control unit is configured to, when a temperature of the catalyst
is higher than or equal to an activation temperature and an
air-fuel ratio of exhaust gas emitted from an engine body is a rich
air-fuel ratio, cause the supply device to supply secondary air
into exhaust gas while periodically increasing or reducing the air
such that the air-fuel ratio of exhaust gas flowing into the filter
alternately varies between rich and lean air-fuel ratios.
Inventors: |
TANAKA; Shogo; (Toki-shi,
JP) ; NAKAGAWA; Norihisa; (Susono-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
67539280 |
Appl. No.: |
16/516778 |
Filed: |
July 19, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N 2560/06 20130101;
F01N 2900/1402 20130101; F01N 3/0231 20130101; F01N 3/22 20130101;
F01N 9/00 20130101; F01N 3/035 20130101; F01N 2560/02 20130101;
F01N 9/002 20130101; F01N 2430/06 20130101; F01N 2900/1404
20130101; F01N 2550/14 20130101 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/22 20060101 F01N003/22; F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2018 |
JP |
2018-155556 |
Claims
1. An exhaust gas control apparatus for an internal combustion
engine, the exhaust gas control apparatus comprising: an exhaust
gas control catalyst disposed in an exhaust passage of the internal
combustion engine, the exhaust gas control catalyst having a
catalytic function; a particulate filter disposed in the exhaust
passage at a location downstream of the exhaust gas control
catalyst in an exhaust gas flow direction; an oxygen supply device
configured to supply gas containing oxygen into exhaust gas flowing
into the particulate filter at a location downstream of the exhaust
gas control catalyst in the exhaust gas flow direction; and an
electronic control unit configured to regulate an amount of oxygen
that is supplied from the oxygen supply device, when a temperature
of the exhaust gas control catalyst falls within a predetermined
temperature range higher than or equal to an activation temperature
and an air-fuel ratio of exhaust gas emitted from a body of the
internal combustion engine is a rich air-fuel ratio richer than a
stoichiometric air-fuel ratio, the electronic control unit being
configured to cause the oxygen supply device to supply oxygen into
the exhaust gas while periodically increasing or reducing the
oxygen such that an air-fuel ratio of exhaust gas flowing into the
particulate filter alternately varies between the rich air-fuel
ratio and a lean air-fuel ratio leaner than the stoichiometric
air-fuel ratio.
2. The exhaust gas control apparatus according to claim 1, wherein
the predetermined temperature range is higher than or equal to
400.degree. C. and lower than or equal to 600.degree. C.
3. The exhaust gas control apparatus according to claim 1, wherein
the electronic control unit is configured to cause the oxygen
supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that an average
air-fuel ratio in a plurality of cycles in which the air-fuel ratio
of exhaust gas flowing into the particulate filter alternately
varies between the rich air-fuel ratio and the lean air-fuel ratio
becomes the stoichiometric air-fuel ratio.
4. The exhaust gas control apparatus according to claim 1, wherein
the electronic control unit is configured to increase a leanness
degree at the time when the air-fuel ratio of exhaust gas flowing
into the particulate filter is the leanest as a temperature of the
particulate filter decreases.
5. The exhaust gas control apparatus according to claim 1, wherein
the electronic control unit is configured to cause the oxygen
supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that the
air-fuel ratio of exhaust gas flowing into the particulate filter
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio with a period shorter than or equal to a period with
which exhaust gas having the lean air-fuel ratio flows into the
particulate filter before exhaust gas having the rich air-fuel
ratio and having flowed into the particulate filter flows out from
the particulate filter and exhaust gas having the rich air-fuel
ratio flows into the particulate filter before exhaust gas having
the lean air-fuel ratio and having flowed into the particulate
filter flows out from the particulate filter.
6. The exhaust gas control apparatus according to claim 1, wherein
the particulate filter has a catalytic function.
7. An exhaust gas control apparatus for an internal combustion
engine, the exhaust gas control apparatus comprising: an exhaust
gas control catalyst disposed in an exhaust passage of the internal
combustion engine, the exhaust gas control catalyst having a
catalytic function; a particulate filter disposed in the exhaust
passage at a location downstream of the exhaust gas control
catalyst in an exhaust gas flow direction; an oxygen supply device
configured to supply gas containing oxygen into exhaust gas flowing
into the particulate filter at a location downstream of the exhaust
gas control catalyst in the exhaust gas flow direction; and an
electronic control unit configured to regulate an amount of oxygen
that is supplied from the oxygen supply device, under a condition
that hydrogen or ammonia is produced in the exhaust gas control
catalyst when an air-fuel ratio of exhaust gas emitted from a body
of the internal combustion engine is a rich air-fuel ratio richer
than a stoichiometric air-fuel ratio, the electronic control unit
being configured to cause the oxygen supply device to supply oxygen
into the exhaust gas while periodically increasing or reducing the
oxygen such that an air-fuel ratio of exhaust gas flowing into the
particulate filter alternately varies between the rich air-fuel
ratio and a lean air-fuel ratio leaner than the stoichiometric
air-fuel ratio.
8. The exhaust gas control apparatus according to claim 7, wherein
the electronic control unit is configured to cause the oxygen
supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that an average
air-fuel ratio in a plurality of cycles in which the air-fuel ratio
of exhaust gas flowing into the particulate filter alternately
varies between the rich air-fuel ratio and the lean air-fuel ratio
becomes the stoichiometric air-fuel ratio.
9. The exhaust gas control apparatus according to claim 7, wherein
the electronic control unit is configured to increase a leanness
degree at the time when the air-fuel ratio of exhaust gas flowing
into the particulate filter is the leanest as a temperature of the
particulate filter decreases.
10. The exhaust gas control apparatus according to claim 7, wherein
the electronic control unit is configured to cause the oxygen
supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that the
air-fuel ratio of exhaust gas flowing into the particulate filter
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio with a period shorter than or equal to a period with
which exhaust gas having the lean air-fuel ratio flows into the
particulate filter before exhaust gas having the rich air-fuel
ratio and having flowed into the particulate filter flows out from
the particulate filter and exhaust gas having the rich air-fuel
ratio flows into the particulate filter before exhaust gas having
the lean air-fuel ratio and having flowed into the particulate
filter flows out from the particulate filter.
11. The exhaust gas control apparatus according to claim 7, wherein
the particulate filter has a catalytic function.
12. An exhaust gas control apparatus for an internal combustion
engine, the exhaust gas control apparatus comprising: a particulate
filter disposed in an exhaust passage of the internal combustion
engine; an oxygen supply device configured to supply gas containing
oxygen into exhaust gas flowing into the particulate filter; and an
electronic control unit configured to control an amount of oxygen
that is supplied from the oxygen supply device, under a condition
that NO.sub.2 is produced with oxygen supplied from the oxygen
supply device when oxygen that is supplied from the oxygen supply
device into exhaust gas is periodically increased or reduced such
that an air-fuel ratio of exhaust gas flowing into the particulate
filter alternately varies between a rich air-fuel ratio richer than
a stoichiometric air-fuel ratio and a lean air-fuel ratio leaner
than the stoichiometric air-fuel ratio, the electronic control unit
being configured to cause the oxygen supply device to supply oxygen
into the exhaust gas while periodically increasing or reducing the
oxygen such that an air-fuel ratio of exhaust gas flowing into the
particulate filter alternately varies between the rich air-fuel
ratio and the lean air-fuel ratio.
13. The exhaust gas control apparatus according to claim 12,
wherein the electronic control unit is configured to cause the
oxygen supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that an average
air-fuel ratio in a plurality of cycles in which the air-fuel ratio
of exhaust gas flowing into the particulate filter alternately
varies between the rich air-fuel ratio and the lean air-fuel ratio
becomes the stoichiometric air-fuel ratio.
14. The exhaust gas control apparatus according to claim 12,
wherein the electronic control unit is configured to increase a
leanness degree at the time when the air-fuel ratio of exhaust gas
flowing into the particulate filter is the leanest as a temperature
of the particulate filter decreases.
15. The exhaust gas control apparatus according to claim 12,
wherein the electronic control unit is configured to cause the
oxygen supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that the
air-fuel ratio of exhaust gas flowing into the particulate filter
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio with a period shorter than or equal to a period with
which exhaust gas having the lean air-fuel ratio flows into the
particulate filter before exhaust gas having the rich air-fuel
ratio and having flowed into the particulate filter flows out from
the particulate filter and exhaust gas having the rich air-fuel
ratio flows into the particulate filter before exhaust gas having
the lean air-fuel ratio and having flowed into the particulate
filter flows out from the particulate filter.
16. The exhaust gas control apparatus according to claim 12,
wherein the particulate filter has a catalytic function.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2018-155556 filed on Aug. 22, 2018 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND
1. Technical Field
[0002] The present disclosure relates to an exhaust gas control
apparatus for an internal combustion engine.
2. Description of Related Art
[0003] An exhaust gas control catalyst for an internal combustion
engine including a secondary air supply device is known (for
example, Japanese Unexamined Patent Application Publication No.
2010-127147 (JP 2010-127147 A) and Japanese Unexamined Patent
Application Publication No. 2007-092713 (JP 2007-092713 A)). The
secondary air supply device supplies secondary air into exhaust gas
flowing through an exhaust passage of the internal combustion
engine. In the exhaust gas control apparatus described in JP
2010-127147 A, a particulate filter (hereinafter, also simply
referred to as filter) is provided in the exhaust passage, and the
secondary air supply device is configured to supply secondary air
into exhaust gas flowing into the filter.
[0004] In the thus configured exhaust gas control apparatus
described in JP 2010-127147 A, when the amount of particulate
matter (hereinafter, also referred to as PM) deposited on the
filter increases, secondary air is supplied while the air-fuel
ratio of exhaust gas that is emitted from an engine body is kept at
an air-fuel ratio richer than a stoichiometric air-fuel ratio
(hereinafter, also referred to as rich air-fuel ratio). Thus, the
secondary air reacts with unburnt fuel contained in the rich
air-fuel ratio exhaust gas on the filter to raise the temperature
of the filter, with the result that the PM is burnt and
removed.
SUMMARY
[0005] Incidentally, it is known that NO.sub.2 contained in exhaust
gas is more reactive with PM than oxygen. For this reason, when
exhaust gas containing a large amount of NO.sub.2 is caused to flow
into the filter, the amount of PM removed can be increased.
However, in the exhaust gas control apparatus described in JP
2010-127147 A, gas composed of rich air-fuel ratio exhaust gas with
a small amount of NO.sub.2 and added secondary air directly flows
into the filter. Therefore, NO.sub.2 is not so contained in the
exhaust gas flowing into the filter, so the burning rate of PM is
not increased.
[0006] On the other hand, as a method of causing exhaust gas
containing a large amount of NO.sub.2 to flow into the filter, it
is conceivable that the air-fuel ratio of exhaust gas that is
emitted from the engine body is set to a lean air-fuel ratio and an
oxidation catalyst is provided upstream of the filter in an exhaust
gas flow direction. Thus, part of NO in exhaust gas emitted from
the engine body is converted into NO.sub.2 by the oxidation
catalyst, the NO.sub.2 is caused to flow into the filter, and PM
deposited on the filter is burnt with the NO.sub.2.
[0007] However, not only NO.sub.2 but also NO is contained in lean
air-fuel ratio exhaust gas flowing into the filter in this case.
When the NO flows into the filter, the NO flows out from the filter
without reacting with PM. Therefore, when the lean air-fuel ratio
exhaust gas is continuously caused to flow into the filter,
emissions of exhaust gas flowing out from the filter
deteriorate.
[0008] The disclosure provides an exhaust gas control apparatus
that is able to reduce deterioration of emissions while
facilitating removal of PM from a filter.
[0009] A first aspect of the disclosure relates to an exhaust gas
control apparatus for an internal combustion engine. The exhaust
gas control apparatus includes an exhaust gas control catalyst, a
particulate filter, an oxygen supply device, and an electronic
control unit. The exhaust gas control catalyst is disposed in an
exhaust passage of the internal combustion engine. The exhaust gas
control catalyst has a catalytic function. The particulate filter
is disposed in the exhaust passage at a location downstream of the
exhaust gas control catalyst in an exhaust gas flow direction. The
oxygen supply device is configured to supply gas containing oxygen
into exhaust gas flowing into the particulate filter at a location
downstream of the exhaust gas control catalyst in the exhaust gas
flow direction. The electronic control unit is configured to
regulate an amount of oxygen that is supplied from the oxygen
supply device. When a temperature of the exhaust gas control
catalyst falls within a predetermined temperature range higher than
or equal to an activation temperature and an air-fuel ratio of
exhaust gas emitted from a body of the internal combustion engine
is a rich air-fuel ratio richer than a stoichiometric air-fuel
ratio, the electronic control unit is configured to cause the
oxygen supply device to supply oxygen into the exhaust gas while
periodically increasing or reducing the oxygen such that an
air-fuel ratio of exhaust gas flowing into the particulate filter
alternately varies between the rich air-fuel ratio and a lean
air-fuel ratio leaner than the stoichiometric air-fuel ratio.
[0010] In the exhaust gas control apparatus, the predetermined
temperature range may be higher than or equal to 400.degree. C. and
lower than or equal to 600.degree. C.
[0011] A second aspect of the disclosure relates to an exhaust gas
control apparatus for an internal combustion engine. The exhaust
gas control apparatus includes an exhaust gas control catalyst, a
particulate filter, an oxygen supply device, and an electronic
control unit. The exhaust gas control catalyst is disposed in an
exhaust passage of the internal combustion engine. The exhaust gas
control catalyst has a catalytic function. The particulate filter
is disposed in the exhaust passage at a location downstream of the
exhaust gas control catalyst in an exhaust gas flow direction. The
oxygen supply device is configured to supply gas containing oxygen
into exhaust gas flowing into the particulate filter at a location
downstream of the exhaust gas control catalyst in the exhaust gas
flow direction. The electronic control unit is configured to
regulate an amount of oxygen that is supplied from the oxygen
supply device. Under a condition that hydrogen or ammonia is
produced in the exhaust gas control catalyst when an air-fuel ratio
of exhaust gas emitted from a body of the internal combustion
engine is a rich air-fuel ratio richer than a stoichiometric
air-fuel ratio, the electronic control unit is configure to cause
the oxygen supply device to supply oxygen into the exhaust gas
while periodically increasing or reducing the oxygen such that an
air-fuel ratio of exhaust gas flowing into the particulate filter
alternately varies between the rich air-fuel ratio and a lean
air-fuel ratio leaner than the stoichiometric air-fuel ratio.
[0012] A third aspect of the disclosure relates to an exhaust gas
control apparatus for an internal combustion engine. The exhaust
gas control apparatus includes a particulate filter, an oxygen
supply device, and an electronic control unit. The particulate
filter is disposed in an exhaust passage of the internal combustion
engine. The oxygen supply device is configured to supply gas
containing oxygen into exhaust gas flowing into the particulate
filter. The electronic control unit is configured to regulate an
amount of oxygen that is supplied from the oxygen supply device.
Under a condition that NO.sub.2 is produced with oxygen supplied
from the oxygen supply device when oxygen that is supplied from the
oxygen supply device into exhaust gas is periodically increased or
reduced such that an air-fuel ratio of exhaust gas flowing into the
particulate filter alternately varies between a rich air-fuel ratio
richer than a stoichiometric air-fuel ratio and a lean air-fuel
ratio leaner than the stoichiometric air-fuel ratio, the electronic
control unit is configured to cause the oxygen supply device to
supply oxygen into the exhaust gas while periodically increasing or
reducing the oxygen such that an air-fuel ratio of exhaust gas
flowing into the particulate filter alternately varies between the
rich air-fuel ratio and the lean air-fuel ratio.
[0013] In the exhaust gas control apparatus, the electronic control
unit may be configured to cause the oxygen supply device to supply
oxygen into the exhaust gas while periodically increasing or
reducing the oxygen such that an average air-fuel ratio in a
plurality of cycles in which an air-fuel ratio of exhaust gas
flowing into the particulate filter alternately varies between the
rich air-fuel ratio and the lean air-fuel ratio becomes the
stoichiometric air-fuel ratio.
[0014] In the exhaust gas control apparatus, the electronic control
unit may be configured to increase a leanness degree at the time
when the air-fuel ratio of exhaust gas flowing into the particulate
filter is the leanest as a temperature of the particulate filter
decreases.
[0015] In the exhaust gas control apparatus, the electronic control
unit may be configured to cause the oxygen supply device to supply
oxygen into the exhaust gas while periodically increasing or
reducing the oxygen such that the air-fuel ratio of exhaust gas
flowing into the particulate filter alternately varies between the
rich air-fuel ratio and the lean air-fuel ratio with a period
shorter than or equal to a period with which exhaust gas having the
lean air-fuel ratio flows into the particulate filter before
exhaust gas having the rich air-fuel ratio and having flowed into
the particulate filter flows out from the particulate filter and
exhaust gas having the rich air-fuel ratio flows into the
particulate filter before exhaust gas having the lean air-fuel
ratio and having flowed into the particulate filter flows out from
the particulate filter.
[0016] In the exhaust gas control apparatus, the particulate filter
may have a catalytic function.
[0017] According to the aspects of the disclosure, an exhaust gas
control apparatus that reduces deterioration of emissions while
facilitating removal of PM in a filter is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Features, advantages, and technical and industrial
significance of exemplary embodiments of the disclosure will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0019] FIG. 1 is a view schematically showing an internal
combustion engine in which an exhaust gas control apparatus
according to a first embodiment is used;
[0020] FIG. 2A is a front view of a filter;
[0021] FIG. 2B is a longitudinal sectional view of the filter;
[0022] FIG. 3A is a view schematically showing reactions that take
place in an exhaust gas control apparatus when a filter
regeneration process is performed;
[0023] FIG. 3B is a view schematically showing reactions that take
place in the exhaust gas control apparatus when the filter
regeneration process is performed;
[0024] FIG. 4 is a timing chart of the amount of secondary air
supplied from a secondary air supply device and the air-fuel ratio
of exhaust gas flowing into the filter;
[0025] FIG. 5 is a flowchart showing a control routine for the
filter regeneration process according to the first embodiment;
[0026] FIG. 6 is a flowchart showing a control routine for a filter
regeneration process according to an alternative embodiment to the
first embodiment;
[0027] FIG. 7 is a graph showing the relationship between a
temperature of the filter and an amplitude;
[0028] FIG. 8 is a graph showing the relationship between the
temperature of the filter and an amplitude; and
[0029] FIG. 9 is a flowchart showing a control routine for a filter
regeneration process according to a second embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0030] Hereinafter, embodiments of the disclosure will be described
in detail with reference to the accompanying drawings. In the
following description, like reference numerals denote similar
components.
First Embodiment
Overall Description of Internal Combustion Engine
[0031] FIG. 1 is a view schematically showing an internal
combustion engine in which an exhaust gas control apparatus
according to a first embodiment is used. As shown in FIG. 1, the
reference numeral 1 denotes an engine body, the reference numeral 2
denotes a cylinder block, the reference numeral 3 denotes a piston
that reciprocates inside the cylinder block 2, the reference
numeral 4 denotes a cylinder head fixed onto the cylinder block 2,
the reference numeral 5 denotes a combustion chamber defined
between the piston 3 and the cylinder head 4, the reference numeral
6 denotes an intake valve, the reference numeral 7 denotes an
intake port, the reference numeral 8 denotes an exhaust valve, and
the reference numeral 9 denotes an exhaust port. The intake valve 6
opens or closes the intake port 7. The exhaust valve 8 opens or
closes the exhaust port 9.
[0032] As shown in FIG. 1, an ignition plug 10 is disposed at the
center of an inner wall surface of the cylinder head 4, and a fuel
injection valve 11 is disposed at an inner wall surface peripheral
portion of the cylinder head 4. The ignition plug 10 is configured
to generate spark in response to an ignition signal. The fuel
injection valve 11 injects a predetermined amount of fuel into the
combustion chamber 5 in response to an injection signal. The fuel
injection valve 11 may be disposed so as to inject fuel into the
intake port 7. In the present embodiment, gasoline having a
stoichiometric air-fuel ratio of 14.6 is used as a fuel.
Alternatively, a fuel other than gasoline or gasoline-mixed fuel
may be used in the internal combustion engine in which the exhaust
gas control apparatus according to the disclosure is used.
[0033] The intake port 7 of each cylinder is coupled to a surge
tank 14 via a corresponding one of intake branch pipes 13. The
surge tank 14 is coupled to an air cleaner 16 via an intake pipe
15. The intake ports 7, the intake branch pipes 13, the surge tank
14, and the intake pipe 15 form an intake passage. A throttle valve
18 is disposed in the intake pipe 15. The throttle valve 18 is
actuated by a throttle valve actuator 17. The throttle valve 18 is
able to change the opening area of the intake passage when the
throttle valve 18 is turned by the throttle valve actuator 17.
[0034] On the other hand, the exhaust port 9 of each cylinder is
coupled to an exhaust manifold 19. The exhaust manifold 19 includes
a plurality of branch portions respectively coupled to the exhaust
ports 9, and a collecting portion that collects these branch
portions. The collecting portion of the exhaust manifold 19 is
coupled to an upstream-side casing 21. An exhaust gas control
catalyst 20 is incorporated in the upstream-side casing 21. The
upstream-side casing 21 is coupled to a downstream-side casing 23
via an exhaust pipe 22. A particulate filter (hereinafter, also
simply referred to as filter) 24 is incorporated in the
downstream-side casing 23. A secondary air supply device 25 is
provided in the exhaust pipe 22 between the exhaust gas control
catalyst 20 and the filter 24. The secondary air supply device 25
supplies secondary air into exhaust gas flowing through the exhaust
pipe 22, that is, exhaust gas flowing into the filter 24. The
exhaust ports 9, the exhaust manifold 19, the upstream-side casing
21, the exhaust pipe 22, and the downstream-side casing 23 form an
exhaust passage.
[0035] The exhaust gas control apparatus of the present embodiment
includes the secondary air supply device 25. Instead, the exhaust
gas control apparatus may include another oxygen supply device as
long as the oxygen supply device is able to supply gas containing
oxygen into exhaust gas flowing into the filter 24. Specific
examples of the oxygen supply device include a device that supplies
only oxygen into exhaust gas.
[0036] An electronic control unit (ECU) 31 is a digital computer,
and includes a random access memory (RAM) 33, a read only memory
(ROM) 34, a microprocessor (CPU) 35, an input port 36, and an
output port 37 connected to one another via a bidirectional bus
32.
[0037] An air flow meter 39 is disposed in the intake pipe 15. The
air flow meter 39 is used to detect the flow rate of air flowing
through the intake pipe 15. An output signal of the air flow meter
39 is input to the input port 36 via an associated AD converter 38.
An upstream-side air-fuel ratio sensor 40 is disposed at the
collecting portion of the exhaust manifold 19. The upstream-side
air-fuel ratio sensor 40 detects the air-fuel ratio of exhaust gas
flowing through the exhaust manifold 19 (that is, exhaust gas
flowing into the exhaust gas control catalyst 20). In addition, a
downstream-side air-fuel ratio sensor 41 is disposed in the exhaust
pipe 22. The downstream-side air-fuel ratio sensor 41 detects the
air-fuel ratio of exhaust gas flowing through the exhaust pipe 22
(that is, exhaust gas flowing out from the exhaust gas control
catalyst 20 and flowing into the filter 24). Output signals of the
air-fuel ratio sensors 40, 41 are also input to the input port 36
via associated AD converters 38.
[0038] The exhaust gas control catalyst 20 is provided with a
catalyst temperature sensor 46. The catalyst temperature sensor 46
is used to detect the temperature of the exhaust gas control
catalyst 20. The filter 24 is provided with a filter temperature
sensor 47. The filter temperature sensor 47 is used to detect the
temperature of the filter 24. A differential pressure sensor 48 is
provided between the exhaust pipe 22 upstream of the filter 24 and
the exhaust pipe 22 downstream of the filter 24. The differential
pressure sensor 48 is used to detect the differential pressure
between the upstream side and downstream side of the filter 24.
Output signals of the temperature sensors 46, 47 and differential
pressure sensor 48 are also input to the input port 36 via
associated AD converters 38.
[0039] A load sensor 43 is connected to an accelerator pedal 42.
The load sensor 43 generates an output voltage proportional to a
depression amount of the accelerator pedal 42. An output voltage of
the load sensor 43 is input to the input port 36 via an associated
AD converter 38. A crank angle sensor 44 generates an output pulse,
for example, each time a crankshaft rotates 15 degrees. The output
pulse is input to the input port 36. The CPU 35 calculates an
engine rotation speed based on the output pulse of the crank angle
sensor 44.
[0040] On the other hand, the output port 37 is connected to the
ignition plugs 10, the fuel injection valves 11, the throttle valve
actuator 17, and the secondary air supply device 25 via associated
driving circuits 45. Therefore, the ECU 31 functions as a
controller that controls the operations of the ignition plugs 10,
fuel injection valves 11, throttle valve actuator 17, and secondary
air supply device 25.
[0041] In the present embodiment, the exhaust gas control catalyst
20 is a three-way catalyst in which catalytic precious metal (for
example, platinum (Pt)) having a catalysis is supported on a
carrier made of ceramics. A three-way catalyst has a function of
removing unburnt HC, CO, and NOx at the same time when the air-fuel
ratio of exhaust gas flowing into the three-way catalyst is kept at
the stoichiometric air-fuel ratio. The exhaust gas control catalyst
20 may be a catalyst other than the three-way catalyst, such as an
oxidation catalyst and an NOx storage-reduction catalyst, as long
as the catalyst has a structure that a substance having a catalysis
is supported.
[0042] FIG. 2A and FIG. 2B are views showing the structure of the
filter 24. FIG. 2A is a front view of the filter 24. FIG. 2B is a
longitudinal cross-sectional view of the filter 24. As shown in
FIG. 2A and FIG. 2B, the filter 24 has a honeycomb structure, and
has a plurality of exhaust gas flow passages 60, 61 extending
parallel to each other. These exhaust gas flow passages include
exhaust gas inflow passages 60 and exhaust gas outflow passages 61.
The downstream end of each exhaust gas inflow passage 60 is closed
by a plug 62. The upstream end of each exhaust gas outflow passage
61 is closed by a plug 63. In FIG. 2A, the hatched portions
represent the plugs 63. Therefore, the exhaust gas inflow passage
60 and the exhaust gas outflow passage 61 are disposed alternately
via a thin partition wall 64. In other words, the exhaust gas
inflow passages 60 and the exhaust gas outflow passages 61 are
disposed such that each exhaust gas inflow passage 60 is surrounded
by the four exhaust gas outflow passages 61 and each exhaust gas
outflow passage 61 is surrounded by the four exhaust gas inflow
passages 60.
[0043] The filter 24 is made of a porous material, such as
cordierite. Therefore, exhaust gas flowing into the exhaust gas
inflow passage 60 passes through the surrounding partition walls 64
as represented by the arrows in FIG. 2B, and flows out to the
adjacent exhaust gas outflow passages 61. In this way, while
exhaust gas is flowing through the partition walls 64, PM contained
in the exhaust gas is trapped by the filter 24.
[0044] The filter 24 has supported catalytic precious metal (such
as platinum (Pt)) having a catalysis. That is, the filter 24 has a
catalytic function. Therefore, the filter 24 is able to not only
trap PM contained in exhaust gas but also oxidize and remove
unburnt HC and CO contained in exhaust gas. The filter 24 may have
another configuration as long as the filter 24 has a supported
substance that traps PM contained in exhaust gas and that has a
catalysis. Furthermore, when an exhaust gas control catalyst having
a catalysis is disposed between the secondary air supply device 25
and the filter 24, the filter 24 does not need to have a supported
substance having a catalysis.
Filter Regeneration Process
[0045] PM trapped in the filter 24 deposits on the filter 24. When
the amount of PM deposited on the filter 24 increases, pores in the
partition walls 64 clog, with the result that a pressure loss of
exhaust gas due to the filter 24 increases. An increase in pressure
loss leads to a decrease in output power and deterioration of
combustion of the internal combustion engine resulting from
difficulty of exhaust gas flow. Therefore, to prevent a decrease in
output power and deterioration of combustion of the internal
combustion engine, when the amount of PM deposited on the filter 24
is greater than a limit deposition amount, PM deposited on the
filter 24 needs to be oxidized and removed. A limit deposition
amount is an amount as follows. When the amount of PM deposited on
the filter 24 increases to the limit deposition amount or above, a
pressure loss due to the filter 24 increases, leading to, for
example, deterioration of the operating state of the internal
combustion engine.
[0046] In the present embodiment, when the amount of PM deposited
on the filter 24 has increased, a filter regeneration process is
performed to oxidize and remove PM. Hereinafter, the filter
regeneration process will be described with reference to FIG. 3A
and FIG. 3B. FIG. 3A and FIG. 3B are views schematically showing
reactions that take place in the exhaust gas control apparatus when
the filter regeneration process is performed. FIG. 3A shows the
case where no secondary air is being supplied from the secondary
air supply device 25. FIG. 3B shows the case where secondary air is
being supplied from the secondary air supply device 25.
[0047] In performing the filter regeneration process, initially,
the temperature of the exhaust gas control catalyst 20 is raised to
its activation temperature or higher. Specifically, the temperature
of the exhaust gas control catalyst 20 is raised to 300.degree. C.
or higher and 700.degree. C. or lower, and is preferably raised to
400.degree. C. or higher and 600.degree. C. or lower.
[0048] In addition, in the present embodiment, in performing the
filter regeneration process, a fuel injection amount from each fuel
injection valve 11 is regulated such that the air-fuel ratio of
exhaust gas that is emitted from the engine body 1 becomes an
air-fuel ratio richer than the stoichiometric air-fuel ratio
(hereinafter, also referred to as rich air-fuel ratio). In other
words, in performing the filter regeneration process, the fuel
injection amount is regulated such that the air-fuel ratio of
exhaust gas flowing into the exhaust gas control catalyst 20
becomes the rich air-fuel ratio. As a result, during the filter
regeneration process, exhaust gas having the rich air-fuel ratio
flows into the exhaust gas control catalyst 20.
[0049] Unburnt HC and CO are contained in exhaust gas having the
rich air-fuel ratio. In addition, since water is produced as a
result of combustion of air-fuel mixture in each combustion chamber
5, water is contained in exhaust gas. Therefore, exhaust gas
containing unburnt HC and CO and water flows into the exhaust gas
control catalyst 20.
[0050] When the temperature of the exhaust gas control catalyst 20
falls within the range of 300.degree. C. to 500.degree. C., as
exhaust gas containing CO and water flows into the exhaust gas
control catalyst 20, the water gas shift reaction expressed by the
following formula (1) takes place in the exhaust gas control
catalyst 20 under the catalysis of the exhaust gas control catalyst
20.
CO+H.sub.2O.fwdarw.H.sub.2+CO.sub.2 (1)
[0051] When the temperature of the exhaust gas control catalyst 20
is higher than or equal to 500.degree. C., as exhaust gas
containing unburnt HC and water flows into the exhaust gas control
catalyst 20, the steam-reforming reaction expressed by the
following formula (2) or formula (3) takes place in the exhaust gas
control catalyst 20 under the catalysis of the exhaust gas control
catalyst 20.
CH.sub.4+H.sub.2O.fwdarw.3H.sub.2+CO (2)
C.sub.12H.sub.26+12H.sub.2O.fwdarw.25H.sub.2+12CO (3)
[0052] Therefore, when the temperature of the exhaust gas control
catalyst 20 is higher than or equal to the activation temperature
(for example, 300.degree. C.), as exhaust gas having the rich
air-fuel ratio flows into the exhaust gas control catalyst 20,
hydrogen is produced in the exhaust gas control catalyst 20.
[0053] Even when the air-fuel ratio of exhaust gas emitted from the
engine body 1 is the rich air-fuel ratio, NOx (mainly, NO) is
contained in the exhaust gas. When the temperature of the exhaust
gas control catalyst 20 is relatively high, NO contained in exhaust
gas in this way reacts with hydrogen into ammonia as expressed by
the following formula (4) in the exhaust gas control catalyst 20
under the catalysis of the exhaust gas control catalyst 20. This
reaction particularly tends to take place when the temperature of
the exhaust gas control catalyst 20 falls within the range of
400.degree. C. to 600.degree. C.
2NO+5H.sub.2.fwdarw.2NH.sub.3+2H.sub.2O (4)
[0054] Therefore, when the temperature of the exhaust gas control
catalyst 20 is higher than or equal to the activation temperature
(particularly, the range of 400.degree. C. to 600.degree. C.), as
exhaust gas having the rich air-fuel ratio flows into the exhaust
gas control catalyst 20, exhaust gas having the rich air-fuel ratio
and containing ammonia flows out from the exhaust gas control
catalyst 20.
[0055] In addition, in the present embodiment, in performing the
filter regeneration process, secondary air is intermittently
supplied from the secondary air supply device 25. FIG. 4 is a
timing chart of the amount of secondary air supplied from the
secondary air supply device 25, and the air-fuel ratio of exhaust
gas flowing into the filter 24. The dashed line X in FIG. 4
represents the air-fuel ratio of exhaust gas before secondary air
is supplied (in the illustrated example, the air-fuel ratio at this
time is 13.6).
[0056] As shown in FIG. 4, in the present embodiment, secondary air
is intermittently supplied from the secondary air supply device 25.
In the example shown in FIG. 4, a constant amount of secondary air
is supplied from time t.sub.0 to time t.sub.1, from time t.sub.2 to
time t.sub.3, from time t.sub.4 to time t.sub.5, and from time
t.sub.6 to time t.sub.7. Particularly, in the present embodiment,
secondary air is intermittently supplied such that a duration (for
example, from time t.sub.0 to time t.sub.1) during which secondary
air is being supplied is equal to a duration (for example, from
time t.sub.1 to time t.sub.2) during which supply of secondary air
is stopped (hereinafter, this duration is also referred to as
period).
[0057] Particularly, in the present embodiment, the switching
period of supply of secondary air is set to at or below a duration
during which exhaust gas having a lean air-fuel ratio flows into
the filter 24 before exhaust gas having the rich air-fuel ratio and
having flowed into the filter 24 flows out from the filter 24
Preferably, the switching period of supply of secondary air is set
to at or below a duration during which exhaust gas having the lean
air-fuel ratio flows into the filter 24 before exhaust gas having
the rich air-fuel ratio and having flowed into the filter 24
reaches a center of the filter 24 in the exhaust gas flow
direction.
[0058] Similarly, the switching period of supply of secondary air
is set to at or below a duration during which exhaust gas having
the rich air-fuel ratio flows into the filter 24 before exhaust gas
having the lean air-fuel ratio and having flowed into the filter 24
flows out from the filter 24. Preferably, the switching period of
supply of secondary air is set to at or below a duration during
which exhaust gas having the rich air-fuel ratio flows into the
filter 24 before exhaust gas having the rich air-fuel ratio and
having flowed into the filter 24 reaches a center of the filter 24
in the exhaust gas flow direction. Specifically, the switching
period of supply of secondary air is set to, for example, about 10
Hz.
[0059] As a result of supply of secondary air from the secondary
air supply device 25 in this way, the air-fuel ratio of exhaust gas
that flows into the filter 24 varies alternately between the rich
air-fuel ratio and an air-fuel ratio leaner than the stoichiometric
air-fuel ratio (hereinafter, also referred to as lean air-fuel
ratio) as shown in FIG. 4. Particularly, in the present embodiment,
secondary air is intermittently supplied such that a richness
degree (for example, a richness degree at time t.sub.0, time
t.sub.2, time t.sub.4, or the like) at which the air-fuel ratio of
exhaust gas flowing into the filter 24 is the richest is equal to a
leanness degree (for example, a leanness degree at time t.sub.1,
time t.sub.3, time t.sub.5, or the like) at which the air-fuel
ratio of exhaust gas flowing into the filter 24 is the leanest.
[0060] As a result, in the present embodiment, an average air-fuel
ratio of exhaust gas flowing into the filter 24 over a certain
length of time is almost the stoichiometric air-fuel ratio. That
is, in the present embodiment, an average air-fuel ratio over
multiple cycles in which the air-fuel ratio of exhaust gas flowing
into the filter 24 varies alternately between the rich air-fuel
ratio and the lean air-fuel ratio is almost the stoichiometric
air-fuel ratio.
[0061] In the present embodiment, secondary air is supplied such
that a richness degree at the time when the air-fuel ratio of
exhaust gas is the richest is equal to a leanness degree at the
time when the air-fuel ratio of exhaust gas is the leanest.
However, these richness degree and leanness degree do not always
need to be equal to each other. Secondary air may be supplied such
that any one of the leanness degree and the richness degree is
higher than the other. In the present embodiment, secondary air is
supplied such that the duration during which secondary air is being
supplied is equal to the duration during which supply of secondary
air is stopped. However, these durations do not always need to be
equal to each other. Secondary air may be supplied such that any
one of the durations is longer than the other. In the present
embodiment, secondary air is supplied such that the average
air-fuel ratio of exhaust gas flowing into the filter 24 is almost
the stoichiometric air-fuel ratio. Instead, secondary air may be
supplied such that the average air-fuel ratio of exhaust gas
flowing into the filter 24 is the rich air-fuel ratio or the lean
air-fuel ratio.
[0062] FIG. 3A shows the reactions that take place in the exhaust
gas control apparatus when no secondary air is being supplied from
the secondary air supply device 25. As is apparent from FIG. 3A,
unburnt HC and CO contained in exhaust gas emitted from the engine
body 1 flow into the filter 24, and ammonia produced in the exhaust
gas control catalyst 20 also flows into the filter 24.
[0063] Part of NOx contained in exhaust gas emitted from the engine
body 1 also remains without being removed in the exhaust gas
control catalyst 20. Therefore, NOx also flows into the filter 24.
NOx having flowed into the filter 24 is reduced and removed by
unburnt HC or CO under the action of the supported catalytic
precious metal in the filter 24. Therefore, when no secondary air
is being supplied, exhaust gas containing unburnt HC, CO, and
ammonia flows to the downstream side of the filter 24.
[0064] On the other hand, FIG. 3B shows the reactions that take
place in the exhaust gas control apparatus when secondary air is
being supplied from the secondary air supply device 25. In a region
upstream of a region where the secondary air supply device 25 is
provided, similar reactions to those in the case where no secondary
air is being introduced take place. Therefore, exhaust gas flowing
into the filter 24 contains unburnt HC, CO, ammonia, and NOx
(particularly, NO). In addition, when secondary air is being
supplied, exhaust gas flowing into the filter 24 contains air,
particularly, oxygen, supplied from the secondary air supply device
25.
[0065] When the temperature of the filter 24 is higher than or
equal to 300.degree. C. at this time, NO.sub.2 is produced from
ammonia and oxygen through the reaction expressed by the following
formula (5) on the filter 24 having supported catalytic precious
metal.
4NH.sub.3+5O.sub.2.fwdarw.4NO.sub.2+6H.sub.2O (5)
[0066] NO.sub.2 produced in this way is more reactive with PM
deposited on the filter 24 than oxygen. Therefore, when the
temperature of the filter 24 is about 300.degree. C., NO.sub.2
oxidizes and removes PM (containing carbon C as a main ingredient)
through the reaction expressed by the following formulae (6) to
(9).
2NO.sub.2+2C.fwdarw.2CO.sub.2+N.sub.2 (6)
NO.sub.2+C.fwdarw.CO+NO (7)
O.sub.2+C.fwdarw.CO.sub.2 (8)
O.sub.2+2C.fwdarw.2CO (9)
[0067] When secondary air is being supplied, unburnt HC and CO
contained in exhaust gas emitted from the exhaust gas control
catalyst 20 react with supplied oxygen on the filter 24, so unburnt
HC and CO are removed. Similarly, CO produced in the above formula
(7) and formula (9) also reacts with supplied oxygen and is
removed. On the other hand, NO contained in exhaust gas emitted
from the exhaust gas control catalyst 20 is not removed and remains
because of a lean atmosphere caused by supply of secondary air.
Therefore, when secondary air is being supplied, exhaust gas
containing NO flows into the region downstream of the filter
24.
[0068] As described with reference to FIG. 2A and FIG. 2B, exhaust
gas flows through the thin partition walls 64 in the filter 24. In
the present embodiment, the air-fuel ratio of exhaust gas flowing
into the filter 24 alternately varies between the rich air-fuel
ratio and the lean air-fuel ratio with a relatively short period.
As a result, even when exhaust gas having the rich air-fuel ratio
and exhaust gas having the lean air-fuel ratio alternately flow in
at the inlet of the filter 24, these exhaust gases are mixed with
each other at the outlet of the filter 24.
[0069] As described above, when no secondary air is being supplied,
exhaust gas containing unburnt HC, CO, and ammonia flows into the
region downstream of the filter 24. On the other hand, when
secondary air is being supplied, exhaust gas containing NO or
oxygen flows into the region downstream of the filter 24. These
exhaust gases mix with each other in the region downstream of the
filter 24, so unburnt HC, CO, and ammonia react with NO and oxygen.
As a result, unburnt HC, CO, ammonia, and NO are removed.
[0070] As described above, in the present embodiment, when
secondary air is being supplied, exhaust gas containing a large
amount of NO.sub.2 flows into the filter 24, so removal of PM
deposited on the filter 24 is facilitated. In addition, in the
present embodiment, exhaust gas having the rich air-fuel ratio and
exhaust gas having the lean air-fuel ratio are caused to
alternately flow into the filter 24 with a short period, so unburnt
HC, CO, NO, and other substances, in exhaust gas are removed. Thus,
deterioration of exhaust emissions is reduced.
[0071] In summary, in the present embodiment, when the temperature
of the exhaust gas control catalyst 20 falls within a predetermined
temperature range higher than or equal to the activation
temperature and the air-fuel ratio of exhaust gas emitted from the
engine body 1 is the rich air-fuel ratio, secondary air (oxygen) is
supplied from the secondary air supply device 25 into the exhaust
gas while being periodically increased or reduced such that the
air-fuel ratio of exhaust gas flowing into the filter 24
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio.
[0072] From another viewpoint, in the present embodiment, under the
condition that hydrogen or ammonia is produced in the exhaust gas
control catalyst 20 when the air-fuel ratio of exhaust gas emitted
from the engine body 1 is the rich air-fuel ratio, secondary air
(oxygen) is supplied from the secondary air supply device 25 into
the exhaust gas while being periodically increased or reduced such
that the air-fuel ratio of exhaust gas flowing into the filter 24
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio.
[0073] From further another viewpoint, in the present embodiment,
under the condition that NO.sub.2 is produced by secondary air
(oxygen) supplied from the secondary air supply device 25 when
oxygen that is supplied from the secondary air supply device 25
into exhaust gas is periodically increased or reduced such that the
air-fuel ratio of exhaust gas flowing into the filter 24
alternately varies between the rich air-fuel ratio and the lean
air-fuel ratio, secondary air (oxygen) is supplied from the
secondary air supply device 25 into the exhaust gas while being
periodically increased or reduced such that the air-fuel ratio of
exhaust gas flowing into the filter 24 alternately varies between
the rich air-fuel ratio and the lean air-fuel ratio.
[0074] According to the present embodiment, when secondary air
supplied from the secondary air supply device 25 is regulated in
this way, deterioration of exhaust emissions is reduced while
facilitating removal of PM deposited on the filter 24.
Specific Control
[0075] Next, specific control in the filter regeneration process
according to the present embodiment will be described with
reference to FIG. 5. FIG. 5 is a flowchart showing the control
routine of the filter regeneration process according to the present
embodiment. The illustrated control routine is executed at set time
intervals.
[0076] First, in step S11, it is determined whether a regeneration
flag is in an on state. The regeneration flag is set to the on
state when filter regeneration is being performed; otherwise, the
regeneration flag is set to an off state. When it is determined in
step S11 that the regeneration flag is in the off state, the
process proceeds to step S12.
[0077] In step S12, it is determined whether a condition for
performing the process of regenerating the filter 24 is satisfied.
The condition for performing the regeneration process is satisfied,
for example, when the amount of PM deposited on the filter 24 is
greater than the limit deposition amount. Specifically, when the
differential pressure between the upstream side and downstream side
of the filter 24, detected by the differential pressure sensor 48,
is greater than a limit differential pressure, it is determined
that the amount of PM deposited on the filter 24 is greater than
the limit deposition amount. On the other hand, when the
differential pressure between the upstream side and downstream side
of the filter 24, detected by the differential pressure sensor 48,
is less than or equal to the limit differential pressure, it is
determined that the amount of PM deposited on the filter 24 is less
than or equal to the limit deposition amount. The amount of PM
deposited on the filter 24 may be detected or estimated by other
methods without using the differential pressure sensor 48.
[0078] When it is determined in step S12 that the condition for
performing the regeneration process is not satisfied, the control
routine is ended. On the other hand, when it is determined in step
S12 that the condition for performing the regeneration process is
satisfied, the process proceeds to step S13. In step S13, the
regeneration flag is set to the on state, and the control routine
is ended.
[0079] When the regeneration flag is set to the on state, the
process proceeds from step S11 to step S14 in the next control
routine. In step S14, it is determined whether a temperature Tc of
the exhaust gas control catalyst 20 is higher than or equal to an
activation temperature Tact (for example, 300.degree. C.). The
temperature of the exhaust gas control catalyst 20 is detected by
the catalyst temperature sensor 46 provided in the exhaust gas
control catalyst 20.
[0080] As described above, when the temperature Tc of the exhaust
gas control catalyst 20 is lower than the activation temperature
Tact, hydrogen or ammonia cannot be produced in the exhaust gas
control catalyst 20. Therefore, when it is determined in step S14
that the temperature of the exhaust gas control catalyst 20 is
lower than the activation temperature Tact, the process proceeds to
step S15, and the process of raising the temperature of the exhaust
gas control catalyst 20 is performed. Examples of the process of
raising the temperature of the exhaust gas control catalyst 20
include dither control in which the air-fuel ratio of air-fuel
mixture that is supplied to the combustion chamber 5 is set to the
rich air-fuel ratio in part of the plurality of cylinders and the
air-fuel ratio of air-fuel mixture that is supplied to the
combustion chambers 5 of the remaining cylinders is set to the lean
air-fuel ratio. Through the dither control, exhaust gas containing
unburnt HC and CO emitted from the cylinders of the rich air-fuel
ratio and exhaust gas containing a large amount of oxygen emitted
from the cylinders of the lean air-fuel ratio mix with each other
and react with each other on the exhaust gas control catalyst 20.
For this reason, the temperature of the exhaust gas control
catalyst 20 is raised by the use of heat of the reaction at this
time. In the process of raising the temperature of the exhaust gas
control catalyst 20, existing temperature raising control other
than dither control may be used instead of dither control.
[0081] When the temperature Tc of the exhaust gas control catalyst
20 rises to the activation temperature Tact or higher as a result
of the process of raising the temperature of the exhaust gas
control catalyst 20, the process proceeds from step S14 to step S16
in the next control routine. In step S16, it is determined whether
the current amount Dpm of PM deposited on the filter 24 is greater
than a minimum allowable value Dmin. The minimum allowable value
Dmin is a predetermined constant value close to zero. As described
above, the amount Dpm of PM deposited on the filter 24 is estimated
based on the differential pressure between the upstream side and
downstream side of the filter 24, detected by the differential
pressure sensor 48 as described above.
[0082] When it is determined in step S16 that the amount of PM
deposited on the filter 24 is greater than the minimum allowable
value Dmin, the process proceeds to step S17. In step S17, rich
air-fuel ratio control for regulating the fuel injection amount
from each fuel injection valve 11 is executed such that the
air-fuel ratio of exhaust gas emitted from the engine body 1
becomes the rich air-fuel ratio. A target air-fuel ratio at this
time is set to, for example, 13.6.
[0083] Subsequently, in step S18, secondary air is intermittently
supplied by the secondary air supply device 25. The period and
amount of supply of secondary air are set such that the air-fuel
ratio of exhaust gas flowing into the filter 24 varies as shown in
FIG. 4. Thus, PM deposited on the filter 24 gradually reduces.
[0084] After that, when the amount Dpm of PM deposited on the
filter 24 reduces and becomes less than or equal to the minimum
allowable value Dmin, the process proceeds from step S16 to step
S19 in the next control routine. In step S19, rich air-fuel ratio
control is stopped, and the air-fuel ratio of exhaust gas that is
emitted from the engine body 1 is set to an air-fuel ratio during
normal operation (for example, the air-fuel ratio is kept around
the stoichiometric air-fuel ratio). Subsequently, in step S20,
supply of secondary air from the secondary air supply device 25 is
stopped. Subsequently, in step S21, the regeneration flag is set to
the off state, and the control routine is ended.
Alternative Embodiment
[0085] Next, an alternative embodiment to the first embodiment will
be described with reference to FIG. 6. In the first embodiment, the
filter regeneration process is performed while a vehicle equipped
with an internal combustion engine is being driven. However, the
filter regeneration process may be performed at a maintenance shop
or another place.
[0086] In this case, the vehicle is provided with an alarm lamp
(not shown) that alarms a driver that the filter regeneration
process is required, and the alarm lamp is connected to the output
port of the ECU 31 via the driving circuit 45. The alarm lamp
lights up when the differential pressure between the upstream side
and downstream side of the filter 24, detected by the differential
pressure sensor 48, becomes greater than the limit differential
pressure. When the alarm lamp lights up, the driver drives the
vehicle to a maintenance shop.
[0087] In this case, the exhaust gas control apparatus for an
internal combustion engine does not need to include the secondary
air supply device 25 when the exhaust pipe 22 has an opening (not
shown) for attaching a secondary air supply device. At the
maintenance shop, a cover attached to the opening of the exhaust
pipe 22 is detached, the secondary air supply device is attached to
this opening, and the secondary air supply device is electrically
connected to the output port of the ECU. After that, the filter
regeneration process is performed.
[0088] FIG. 6 is a flowchart showing the control routine of the
filter regeneration process according to the present alternative
embodiment. The illustrated control routine is executed at set time
intervals after the secondary air supply device 25 is attached.
[0089] First, it is determined in step S31 whether the temperature
Tc of the exhaust gas control catalyst 20 is higher than or equal
to the activation temperature Tact. When the temperature Tc is
lower than the activation temperature Tact, the process proceeds to
step S32, and the process of raising the temperature of the exhaust
gas control catalyst 20 is performed. It is conceivable that the
process of raising the temperature of the exhaust gas control
catalyst 20 is, for example, attaching an electric heater around
the exhaust gas control catalyst 20 and then supplying electric
power to the electric heater.
[0090] After that, when the temperature Tc of the exhaust gas
control catalyst 20 rises to the activation temperature Tact or
higher, the process proceeds from step S31 to step S33 in the next
control routine. Step S33 to step S37 are basically similar to step
S16 to step S20 of FIG. 5, so the description thereof is
omitted.
Second Embodiment
[0091] Next, an exhaust gas control apparatus for an internal
combustion engine according to a second embodiment will be
described with reference to FIG. 7. The configuration and control
of the exhaust gas control apparatus according to the second
embodiment are basically similar to the configuration and control
of the exhaust gas control apparatus according to the first
embodiment, so portions different from the exhaust gas control
apparatus according to the first embodiment will be mainly
described below.
[0092] Incidentally, where a leanness degree at the time when the
air-fuel ratio of exhaust gas flowing into the filter 24 is the
leanest is referred to as amplitude when secondary air is being
intermittently supplied, the amount of PM that is removed per unit
time in the filter regeneration process (PM removal amount) varies
with the amplitude. This will be described with reference to FIG.
7.
[0093] FIG. 7 is a graph showing the relationship between an
amplitude and a temperature of the filter 24. In the graph, Tact
denotes the activation temperature (for example, 300.degree. C.) of
the filter 24, Tpm denotes a lowest temperature (for example,
500.degree. C.) at or above which PM can be removed by the use of
oxygen, and Tot denotes a limit temperature (for example,
950.degree. C.) at or above which the filter 24 breaks because of
melting, or the like.
[0094] The plurality of solid lines in the graph represent equal
removal amount lines on each of which the PM removal amount is
equal. Among the plurality of solid lines, the solid line located
on the upper side represents the equal removal amount line for a
greater PM removal amount. Therefore, as is apparent from FIG. 7,
when the temperature of the filter 24 is constant, the PM removal
amount increases as the amplitude increases. When the amplitude is
constant, the PM removal amount increases as the temperature of the
filter 24 rises. As the temperature of the filter 24 rises, the
same amount of PM can be removed by a smaller amplitude.
[0095] The region A in the graph represents a region as follows.
When the filter regeneration process is continued in a state within
the region, the temperature of the filter 24 finally becomes higher
than or equal to the limit temperature Tot. Therefore, when the
filter regeneration process is performed, the amplitude needs to be
regulated so as not to become the state within the region A.
[0096] Even when the temperature of the filter 24 is lower than the
lowest temperature Tpm, exhaust gas having the rich air-fuel ratio
and exhaust gas having the lean air-fuel ratio alternately flow
into the filter 24 through the filter regeneration process. With
this, unburnt HC and CO contained in exhaust gas react with oxygen.
Because of the heat of reaction, the temperature of the filter 24
rises to the lowest temperature Tpm or higher. The region B in the
graph represents a region as follows. Even when the filter
regeneration process is continued in a state within the region, the
temperature of the filter 24 does not rise to the lowest
temperature Tpm or higher. In addition, the region C in the graph
represents a region as follows. Even when the filter regeneration
process is continued in a state within the region, no reaction of
unburnt HC and CO with oxygen occurs on the filter 24, so the
temperature of the filter 24 does not rise to the activation
temperature Tact or higher. Therefore, when the filter regeneration
process is performed, the amplitude needs to be regulated such that
the state does not fall within the region B or the region C.
[0097] In the present embodiment, the leanness degree (amplitude)
at the time when the air-fuel ratio of exhaust gas flowing into the
filter 24 is the leanest is varied with the temperature of the
filter 24. FIG. 8 is a graph showing the relationship between a
temperature of the filter 24 and an amplitude. As shown in FIG. 8,
in the present embodiment, basically, as the temperature of the
filter 24 decreases, the amplitude is increased. Thus, while the PM
removal amount per unit time is kept at a large amount, an
excessive increase in the temperature of the filter 24 is
inhibited.
[0098] If the amplitude is excessively increased, when the final
average air-fuel ratio is intended to be set to almost the
stoichiometric air-fuel ratio, the richness degree at the time when
the air-fuel ratio of exhaust gas flowing into the filter 24 is the
richest needs to be increased. However, if the air-fuel ratio of
air-fuel mixture that is supplied to each combustion chamber 5 is
excessively decreased, it leads to deterioration of combustion, so
the richness degree cannot be increased so much.
[0099] In the case where the richness degree at the time when the
air-fuel ratio of exhaust gas flowing into the filter 24 is the
richest is not varied, if the amplitude is excessively increased,
the duration during which the air-fuel ratio of exhaust gas flowing
into the filter 24 is the rich air-fuel ratio needs to be
excessively extended when the final average air-fuel ratio is
intended to be set to almost the stoichiometric air-fuel ratio.
However, if the duration is excessively extended so much, exhaust
gas having the rich air-fuel ratio and exhaust gas having the lean
air-fuel ratio do not mix with each other in the filter 24.
[0100] For the above reasons, in the present embodiment, as shown
in FIG. 8, the amplitude is increased with a decrease in the
temperature of the filter 24, and, when the amplitude reaches a
predetermined amplitude .DELTA.Lref, the amplitude is kept at the
predetermined amplitude .DELTA.Lref in the range in which the
temperature of the filter 24 is lower than the temperature at the
predetermined amplitude .DELTA.Lref.
[0101] FIG. 9 is a flowchart showing the control routine of the
filter regeneration process according to the present embodiment.
The illustrated control routine is executed at set time intervals.
Step S41 to step S47 in FIG. 9 are similar to step S11 to step S17
in FIG. 5, and step S49 to step S52 in FIG. 9 are similar to step
S18 to step S21 in FIG. 5, so the description thereof is
omitted.
[0102] When rich air-fuel ratio control is executed in step S47, a
target value of the amplitude is subsequently calculated in step
S48. A target value of the amplitude is, for example, calculated
based on the temperature of the filter 24, detected by the filter
temperature sensor 47, by consulting a map as shown in FIG. 8.
Subsequently, in step S49, secondary air is intermittently supplied
by the secondary air supply device 25 such that the amplitude
becomes the target value calculated in step S48. When the richness
degree of the air-fuel ratio of exhaust gas that is emitted from
the engine body 1 is varied with the amplitude, the fuel injection
amount that is injected from each fuel injection valve 11 is also
regulated for the amplitude calculated in step S48.
* * * * *